In U.S. Pat. No. 166,761 there is described an encryption apparatus which comprises a square plate which is divided by an even number of columns and an equal number of rows to form a grid of squares. Openings are provided in one quarter of the squares, through each of which openings a character forming part of a message can be written. A board having a grid of squares constructed as above will have a rotational axis locating at the point of intersection of the bisectors of adjacent sides of the grid, which bisectors in effect divide the board into four notional quadrants. The position of the openings is selected so that if it is considered that the quadrants were independently rotatable about the notional axis so as to be superposed, no opening would coincide with any other opening. Otherwise expressed, the openings have a perfect rotational asymmetry on the board.
The board is used by writing a character at a time through the openings, which in the illustrative example of the patent are one hundred and forty four in number. The board is then rotated about its rotational axis through ninety degrees (in the patent a locating holder being used) and the next group of one hundred and forty four characters are written, and so on until the board has been rotated through one complete revolution.
The columns and rows of the board are each numbered sequentially, whereby the coordinates of any opening can be readily identified for future reference or for transmission to another person for use in decoding the message. The number of different combinations of openings is very high even where the grid of squares is relatively small, and considerable difficulty arises in ensuring the condition of perfect asymmetry of the location of the openings where this is attempted in a haphazard manner. Moreover, there is no provision for changing the location of the openings in a board. Additionally, the method requires the use of a tool for cutting openings in the board.
In U.S. Pat. No. 156,851 an encryption board which is generally of a similar nature to that described above but which includes indicia repeated in a rotationally symmetrical manner in each quadrant, and the patentee describes a method of selecting the position of openings that will ensure a perfect rotational asymmetry. The method involves making up a table, transferring information from the table to the board, and then cutting openings through the board. This method remains relatively time consuming, and does not overcome the others of the above mentioned drawbacks.